Covalent bond fission – Homolytic and heterolytic

Organic reactions usually involve making and breaking of covalent bonds. The fission of bonds can take place in two ways.

Bond breaking is also known as bond fission.

1. Homolytic fission
2. Heterolytic fission
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Heterolytic fission results in the formation of two different chemical species in the sense that one is a cation and the other an anion.

Homolytic fission results in two electrically uncharged radicals.

Radicals have an unpaired electron.
Radicals are particles that have an unpaired electron. They may be single atoms (e.g. chlorine radical, Cl.) or groups of covalently bonded atoms (e.g. methyl radical,.CH3).

Some radicals, called biradicals, have two unpaired electrons, for example, .O. (1s22s22p4) and .O2..

Because of the unpaired electron, radicals can be very reactive. However, there are some that are relatively stable and behave somewhat like ordinary molecules. An example is nitrogen monoxide.

Heterolytic fission versus Homolytic fission...

The hydrogen chloride molecule (H-Cl) is polar owing to the greater electronegativity of the chlorine atom. Heterolytic fission is more common where a chemical bond is already polar. Hydrogen chloride is highly soluble in water and becomes fully ionised; it is a strong acid. Solvents with polar molecules favour heterolytic fission.

Homolyic fission is favoured by non-polar solvents, or by gaseous conditions, and the presence of visible or ultraviolet light.

http://www.avogadro.co.uk/light/fission/bondfission.htm
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Reaction Intermediates

The species produced during cleavage of bonds are called reaction intermediates. The important ones are:

1. Free radical: A free radical is an atom or group of atoms having an unpaired electron. Thee are produced during the homolytic fission of a covalent bond. These are very reactive. The free radicals are classified as primary, secondary or tertiary depending upon whether one, two or three carbon atoms are attached to the carbon atom carrying the odd electron.

The order of stability of alkyl free radicals is

cH3<1 br="">
2. carbocation: It is a group of atoms which contain positively charged carbon having only six electrons. It is obtained by heterolytic fission of covalent bond involving carbon atoms.

Relative stabilty: The methyl group has +I inductive effect. So the alkyl group (as a specific example methyl group)attached to +vely charged carbon (carbocation) tends to releaase electrons towards carbon. As a result, it decreases +charge on the carbocation. Due to this inductive effect, the positive charge on the carbocation gets reduced and dispersed (Dispersion is distribution of charge over other atoms in the molecule). The dispersal of charge results into stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.

So the order of stabilty is

CH3^+<1 br="">
3. Carbanion: It is a species containing a carbon atom carrying a negative charge. They are generated during heterolytic fission of covalent bonds containing carbon, when an atom linked to carbon goes without the bonding electrons.

Carbanions are very reactive species.

They are classified as primary, secondary and tertiary.

The order of stability is reverse of that of carbocations and free radicals.

CH3‾ >1°>2°>3°

4. carbene: The carbenes are reactive neutral species in which the carbon atom has six electrons in the valence shell out of which two are shared. The simplest carbene is methylene (:CH2). It is formed wbehg diazomethan is decomposed by the action of light.

CH2N2 --> :CH2 + N2

Types of attacing reagents

1. Free radicals
2. Electrophiles
3. Nucleophiles


Typesof organic reactions

1. substitution reactions
2. Addition reactions
3. Elimination reactions
--i) α-Elimination
--ii) β-Elimination
--iii)γ-Elimination
4. Rearrangement reactions
5. Condensation reactions
6. Isomerism reactions

Isomerism - Structural isomerism

Isomerism: Organic compounds exhibit isomerism. Isomerism is a phenomenon that describes the fact two compounds having same molecular formula have different physical and chemical properties.
12. Structural isomerism and stereo isomerism are two major categories in isomerism.

Homologous Series

Homologous Series: A series of similarly constituted compounds containing the same functional group and have similar chemical charateristics.

Functional Groups of Organic Compounds

Functional group: Functional groups of structural features within a molecule that determine its reactivity. Thus a functional group is a group of atoms within a molecule that has a charateristic chemical behaviour. For illustration, the simplest functional group is the carbon-carbon double bond. Similarly carbon-carbon triple bond will be another functional group.


The important functional groups are:
Alkyl halide,
Alcohols,
Ethers,
Amines,

Thiol,
 Aldehyde,
Ketone,
Carboxylic acids,
Esters,
Acid halides,
Amides,
Nitriles.

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http://www.youtube.com/watch?v=t4lIp0Z1mVg


Updated on 11 Jan 2020
17 December 2011

Skeletal Structures of Organic Molecules - Video

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http://www.youtube.com/watch?v=RP6AS7XVIC8

IUPAC Nomenclature of Simple Organic Compounds - Video

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http://www.youtube.com/watch?v=hXcrQn3rUhY

Organic Chemistry - Basic Concepts, Points, and Principles

1. Modern definition of the organic chemistry: Chemistry of the hydrocarbons and their derivatives.

2. Catenation: property of an atom to form bonds with atoms of the same element is called catenation. Carbon shows maximum catenation in its group (group 14) in the periodic table. Hence carbon atoms form linear chains, branched chains and rings of different sizes.

3. Sigma and Pi bonds: When there is a double bond between two carbon atoms, one is a sigma bond and one is Pi bond.

4. Organic compounds are represented by structural formula, bond line structural representation.
5. Three dimensional representations of organic compounds include solid and dash wedged formula, Fischer projection, Newmann projection, Sawhorse projection formulae etc.
6. Models can be employed to visualize bonds of organic compounds. There are framework models, ball and stick model, and space filling model.
7. Functional group: Functional groups of structural features within a molecule that determine its reactivity. Thus a functional group is a group of atoms within a molecule that has a charateristic chemical behaviour. For illustration, the simplest functional group is the carbon-carbon double bond. Similarly carbon-carbon triple bond will be another functional group.
8. The important functional groups are:
Alkyl halide, Alcohols, Ethers, Amines, Thiol, Aldehyde, Ketone, Carboxylic acids, Esters, Acid halides, Amides, Nitriles.
9. Homologous Series: A series of similarly constituted compounds containing the same functional group and have similar chemical charateristics.
10.Nomenclature of organic compounds: There are two systems. In trivial system, the organic compounds were named after the source from which they were obtained and other bases are also used. There is no systematic basis. IUPAC (International Union of Pure and Applied Chemistry) developed a system of nomenclature.
11. Isomerism: Organic compounds exhibit isomerism. Isomerism is a phenomenon that describes the fact two compounds having same molecular formula have different physical and chemical properties.
12. Structural isomerism and stereo isomerism are two major categories in isomerism.
13. Benzene ring: Benzene is an organic compound that is represented as having six carbon atoms in the form of a hexagon with three double bonds in the alternative positions. It molecular formula is C6H6.
14. Aromatic compounds are those which contain one or more benzene rings in them. Aromatic compounds have two main parts. Nucleus, the benzene ring and an alkyl or aliphatic group containing at least one carbon atom attached to the nucleus.
15. The organic compounds which contain only hydrogen and carbon are hydrocarbons.
16. Hydrocarbons are broardly divided into four types. 1. Alkanes 2. Alkenes 3. Alkynes 4. Arenes
17. In alkanes, there are only carbon-carbon single bonds. Both open chain and closed chain (ring) alkanes are there.
18. In alkenes, there are carbon-carbon double bonds apart from some double bonds.
19. In alkynes, there are carbon-carbon triple bonds apart from single or double bonds.
20. Arenes have at least one special type of hexagonal ring of carbon atoms with three double bonds and three single bonds in alternative positions. The ring is called benzene ring.


To be continued and more points to be added.